Electrohydraulic proportional actuators are commonly used to generate pilot pressure for actuating pilot actuated hydraulic control valves, clutches, brakes and the like. The generated pilot pressure normally acts against the fluid metering element of the proportional actuator so that the pilot pressure is proportional to the current used to energized the coil of the actuator. The perceived belief that proportional actuators should be a screw-in cartridge style and have a dry removable coil for serviceability increases the cost of the actuators. For example, screw-in cartridge style actuators require housing threads, hydraulic adapter threads and coil retention threads. Threads in general are costly to manufacture and generate a potential source of contamination to the hydraulic system. Also, such actuators typically have three movable parts, i.e. an armature, a pin and a ball or spool, and several parts that are brazed or welded together. Moreover providing a dry removable coil lowers the electromagnetic efficiency and increases the overall size of the actuator due to the existence of an air gap between the coil and the armature. The air gap results from the necessity of providing clearance to put the coil over the solenoid tube housing the armature and the pin. Finally, some proportional actuators have fixed small diameter orifices to minimize flow loses. However those small orifices causes contamination sensitivity and reduces response time.
Thus it is believed that the overall cost of electrohydraulic proportional actuators can be reduced by having no serviceable or replaceable coils, no threads, no welded parts, no small fixed orifices and only one moving part. It is also believed that the electromagnetic efficiency can be increased by deeply integrating the coil into the housing.